The present invention relates generally to a method and apparatus of determined location of a patient on a support surface and more specifically, to its application to an exit alarm system.
The patient exit alarm systems vary from a very simple sensor switch which determines when the patient is not in the center of the bed to various sophisticated systems which determines the center of gravity of the weight from various sensors about the periphery of the support surface. While the simple sensor switch is inexpensive, the calculation of the center of gravity requires a microprocessor or other digital computing device and therefore, is relatively expensive.
A system in between the two systems just described, includes four comer sensors and a center sensor is illustrated in FIG. 10. The change in value of all of the five sensors are determined and various combinations of these changes are used to determine whether the patient has exited the bed or whether the patient is getting ready to exit the bed. While the exit determination is made on the change of the total weight, or changes in the weight of the center sensor in combination with the changes of the other sensors, the prediction of an exit is determined from changes in the center sensor with also large changes in one of the other four sensors. Such a system is described in U.S. patent application Ser. No. 09/031,749 filed Feb. 24, 1998 to inventor Tom Scott.
Systems using a calculation of the center of gravity using signals from a plurality sensor or load cells generally assumes that all the sensors are identical and calibrated to the same parameters. Also, their installation or positioned relative to each other affects its accuracy. The systems which make determinations of changes of signals from a sensor minimizes the calibration of positioning error introduced in the center of gravity calculation systems.
The present invention is a bed system for determining the location of a patient on a support surface. The system includes a first and second sensor coupled to the patient support surface and being at least laterally spaced. A first load signal from the first sensor has a first weight greater than a second weight of a second load signal from the second sensor. A determination circuit receives a first and second load signals and provides an indication of the location of the patient on the support surface from a comparison of the sum of the load signals to a selected value.
The first and second sensors may also be longitudinally spaced. If a third sensor is used, it is longitudinally spaced from the first sensor and its load signal has a third weight less than the first weight. If a fourth sensor is provided, it is longitudinally spaced from the second sensor and laterally spaced from the third sensor and its load signal has a fourth weight which is less than the first weight. The third and fourth weights may be greater than or equal to the second weight and may be equal to each other or different from each other.
The signal processing circuit is provided for setting the weight of the loads. In an analog system, the signal processing or weight setting circuit includes impedance for setting the weight of the load signals. In a digital system, the signal processing is a programmable signal processor and the weights are set by programming. Also, the determination circuit may be part of the digital or programmable signal processor.
The selected value for the comparison may be the sum taken when the patient is substantially centered on the support surface. The selected value may also be selected based on the weight of the patient or based on the range of weight in which the weight of the patient lies. The determination circuit may also provide an indication of location of the patient on the support surface by comparing the sum of the load signals to a plurality of selected values. The determination circuit provides an indication of location of the patient on the support surface when the comparison of the sum of the load signals to the selected value is greater than a selected amount. An alarm indication may also be provided when the comparison is greater than the selected amount or when the location is determined that the patient is adjacent the edge of the support surface.
A method of the present invention for determining the location of the patient on the support surface have load sensors, the method comprises weighting a first load signal of the first sensor to a first weight and weighting the second load signal of the second sensor to a second weight less than the first weight. The weighed load signals are compared to a selected value. The location of the patient on the support surface is determined from the results of the comparison.
The selected value is set to the sum taken when the patient is substantially centered. Alternatively, the selected value may be set based on the weight of the patient or on the range of weight in which the weight of the patient lies. The location of the patient can also be determined from comparing the sum of the load signals to a plurality of selected values. Also, the location can be determined when the comparison of the sum of the loaded values to the selected value is greater than a selected amount. An alarm indication can be provided when the location of the patient on the support surface is adjacent the edge of the support surface. If multiple sensors are provided, they are also weighted to a weight less than the weight of the first load signal.
Other advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.